1. Field of the Invention
The present invention relates to ways and means for eliminating particulate pollutants from combustion gases and, more particularly, to a method and device for removing soot from exhaust gases of internal combustion engines, especially diesel engines, wherein the particles are filtered from the exhaust gas flow and combusted with the aid of a combustion-inducing substance.
2. Description of the Prior Art
The exhaust gases of certain internal combustion engines, especially of the compression-ignited diesel-type, contain unburned carbon particles in the form of soot which, when allowed to discharge into the atmosphere, represent an air pollutant.
In an effort to remove this air pollutant, it has already been suggested to arrange in the exhaust system of the internal combustion engine a soot filter with a filter cartridge of highly temperature-resistant material which collects and retains the soot. However, as more and more soot accumulates on the surface of the filter cartridge during operation of the engine, the flow resistance of the filter cartridge increases to a point where the exhaust flow is substantially blocked, or small chunks of soot are torn through the filter cartridge to create flow channels, thereby defeating and destroying the soot filter.
The need for the periodic removal of the accumulated soot from the exhaust filter has already led to the suggestion of combusting the soot layer on the filter cartridge. This very attractive method of soot removal, however, requires not only the presence of oxygen for combustion, but also temperature levels which are not regularly present, at least not under idling or partial-load operating conditions of the internal combustion engine. It then becomes necessary to provide a supplemental heat source to obtain the necessary combustion temperature.
Such a supplemental heat source is suggested in the German Offenlegungsschrift (Publ. Application) No. 27 56 570 (corr. to U.S. Appln. Ser. No. 779,372 of Mar. 21, 1977) which discloses a heatable exhaust filter for a fuel-injected internal combustion engine. A filter bed of clay-covered steel wool collects the carbon particles from the exhaust gas, and an electrical heater upstream of the filter bed supplies supplemental heat for the combustion of the accumulated soot. This device requires a comparatively large amount of electrical energy for its heater, because the heat is not imparted to the carbon particles directly, but to the exhaust gases which transmit only a portion of the heat to the soot on the filter bed, while carrying a large portion of the heat to the exhaust outlet.
In the German Offenlegungsschrift No. 28 15 365 (corr. to U.S. Appln. Ser. No. 811,666 of June 30, 1977) is disclosed another suggestion of an exhaust filter with an auxiliary heat source, the latter being in the form of a burner which is periodically supplied with compressed propane gas and air. As in the previously mentioned example, the supplemental heat is carried to the accumulated soot by the exhaust gas, so that a comparatively large amount of supplemental energy is needed to combust the soot. The filter itself has to be of very highly heat-resistant materials to withstand the localized high temperatures which result from such an arrangement. A separate tank for the heating gas and a source of supply for the latter are necessary.
Another soot filter for fuel-injected internal combustion engines is suggested in the German Offenlegungsschrift No. 27 50 960. In order to utilize the highest possible exhaust temperature, this filter is arranged immediately downstream of the exhaust manifold. It has a filter element of ceramic fibers, intended to produce the effect of a heat sink, in order to achieve the soot combustion temperature. It has been found, however, that under a range of load conditions, the temperature levels obtained with this device are insufficient for a reliable soot removal.